1. Technical Field Of The Invention
The present invention is a continuation of U.S. application Ser. No. 08/239,382 filed May 6, 1994, which is directed to systems and methods for developing a set of modular fixture designs for a workpiece and, more particularly, the present invention is directed to developing the set of all admissible fixture designs that exists and finding the optimal fixture design based on a user defined quality metric.
2. Description Of Related Art
Currently, for automated manufacturing assembly and inspection operations, which require fixtures to locate and hold workpieces, design fixtures for the shape, position and orientation of the workpiece are generally custom designed by manufacturing engineers and machinists. Presently, general guidelines, such as the 3-2-1 rule, are used and processes for automatically developing design fixtures based on CAD workpiece models are still lacking. The absence of automated design fixture systems is largely due to the large amount of alternative fixture designs that must be considered in most cases. Modular fixture systems are known that typically include a square lattice of tapped and dowel holes with spacings tolerance to .+-.0.002 inches and an assortment of precision locating and clamping elements that can be rigidly attached to the lattice using dowel pins or expanding mandrels. Such modular fixture systems greatly reduce the number of alternatives but human intuition and trial and error techniques are still necessary to design the modular fixture. Furthermore, in the present modular fixturing systems, an acceptable design may not be found or a sub-optimal fixture design may be used because a complete set of all admissible fixture designs are not systematically considered.
Known processes find a set of four (seven) independent regions on the boundary of a polygon (polyhedron) such that a frictionless contact applied to each region is guaranteed to provide form closure, which is a kinematic constraint condition that prevents motion of the workpiece. The set of four or seven independent regions are useful because of uncertainties in the pose of the workpiece before grasping is allowed. It is also known that comparable regions on a polygon can be found by using linear optimization with three frictional contacts in the plane. Also, processing for re-orienting a polygon through a series of form closure grasping operations is known.
Further methods for analyzing the performance of a given fixture have also been analyzed. An Automatically Reconfigurable Fixture (ARF) system automatically synthesizes and constructs work holdings consisting of modular fixtures and determines whether a given fixture and design provides total constraint of a rigid body as well as loading accessibility before clamping. However, the ARF system fails to consider geometric access constraints in addition to kinematic closure.
A method of evaluating the degree of motion constraint imposed by seven fixels (fixture elements which refer to either a locator or a clamp on a fixture plane) contacting a three dimensional rigid body is also known. In this method, an extension of screw theory is applied based on motion stops for producing a metric estimate of the quality of the kinematic constraint of a given fixture design. However, a quality metric based on a user specified set of expected applied forces is not provided by this method.
It is desired to provide an automatic fixture design system which explicitly considers the entire process plan as well as non-geometric effects such as part deformation, tolerances, the release of internal stress during machining and the ability to flush away chips during cutting for example. The present invention is directed to providing a complete system and method that develops all admissible design fixtures so that the fixture design best satisfying the kinematic closure and geometric access constraints may be selected.